Correlated double sampling (CDS) circuit for decreasing settling time and image sensor including the same

ABSTRACT

A correlated double sampling (CDS) circuit includes a comparator and a first circuit. The comparator including, a first input terminal, a second input terminal, at least one output terminal, and a plurality of first transistors operably coupled between the at least one output terminal and the first and second input terminals. The first circuit includes at least one second transistor, the at least one second transistor operably coupled to the at least one output terminal and one of the first input terminal and the second input terminal, the at least one second transistor having at least one of (i) a different number of layers than the first transistors, and (ii) a different dimension than the first transistors.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional application of and claims priorityunder 35 U.S.C. § 120/121 to U.S. application Ser. No. 15/438,060, filedFeb. 21, 2017, which claims the benefit of Korean Patent Application No.10-2016-0041829, filed on Apr. 5, 2016, in the Korean IntellectualProperty Office, each of which are hereby incorporated by referenceherein in their entirety.

BACKGROUND

One or more example embodiments of inventive concepts relate to acorrelated double sampling (CDS) circuit such as a CDS circuitconfigured to decrease a settling time and an image sensor including thesame.

A single-slope analog-digital converting method has been widely used asan analog-digital converting method in the field of image sensor.

In this method, a ramp signal and a pixel signal having a certainvoltage are compared with each other and a time or a point of time atwhich a voltage of the ramp signal and a voltage of the pixel signal areequal to each other is converted into a digital signal on the basis of aresult of comparing these signals with each other.

In a column parallel analog-digital converting method, one or morecolumn analog-digital converters (ADCs) should be integrated in onepixel pitch. Thus, the single-slope analog-digital converting method hasbeen widely used in consideration of layout area and power consumption.

An image sensor employs CDS, and counts a signal sampled through CDS,e.g., the difference between a reset signal and an image signal, andoutputs a digital signal.

Recently, high-resolution and high-frame-rate (HFR) driving technologieshave been used in the field of complementary metal-oxide semiconductor(CMOS) image sensor (CIS). In particular, when an image of a fast movingobject is captured, performing an HFR operation of 120 fps (frame persecond) or more is very important to suppress image distortion caused bya rolling shutter of a CIS.

For the HFR operation, a technique of decreasing a pixel settling timeduring an operation of an ADC of an image sensor has been suggested.

SUMMARY

According to an aspect of inventive concepts, a correlated doublesampling (CDS) circuit includes a comparator configured to compare apixel signal and a ramp signal with each other and output a comparisonsignal corresponding to a result of comparing the pixel signal and theramp signal with each other, and a first switch and a second switch,each of the first switch and the second switch configured to remove anoffset component of the comparator. The first switch and the secondswitch are configured to decrease a settling time between when switchingoperations of the first switch and the second switch are performed andwhen analog-digital conversion (ADC) is performed.

According to another aspect of inventive concepts, there is provided animage sensor including a pixel array having a plurality of pixels foroutputting a pixel signal, a ramp signal generator for outputting a rampsignal, and a correlated double sampling (CDS) circuit. The CDS circuitincludes a first capacitor configured to receive a pixel signal, asecond capacitor configured to receive a ramp signal, a comparatorconfigured to compare the pixel signal and the ramp signal with eachother and output a comparison signal corresponding to a result ofcomparing the pixel signal and the ramp signal with each other, and afirst switch and a second switch, each of the first switch and thesecond switch configured to remove an offset component of thecomparator. The first switch and the second switch are configured todecrease a settling time between when switching operations of the firstswitch and the second switch are performed and when analog-digitalconversion (ADC) is performed. The first switch and the second switchare transistors to which a silicon-germanium (cSiGe) process is notapplied.

According to at least one example embodiment, a correlated doublesampling (CDS) circuit includes a comparator and a first circuit. Thecomparator including, a first input terminal, a second input terminal,at least one output terminal, and a plurality of first transistorsoperably coupled between the at least one output terminal and the firstand second input terminals. The first circuit includes at least onesecond transistor, the at least one second transistor operably coupledto the at least one output terminal and one of the first input terminaland the second input terminal, the at least one second transistor havingat least one of (i) a different number of layers than the firsttransistors, and (ii) a different dimension than the first transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments of inventive concepts will be more clearlyunderstood from the following detailed description taken in conjunctionwith the accompanying drawings in which:

FIG. 1 is a schematic block diagram of an image sensing system includingan image sensor according to an example embodiment of inventiveconcepts;

FIG. 2 is a detailed block diagram of the image sensor of FIG. 1;

FIG. 3 is a detailed block diagram of a correlated double sampling (CDS)circuit according to some example embodiments of inventive concepts;

FIG. 4 is a more detailed circuit diagram of a CDS circuit according tosome example embodiments of inventive concepts;

FIG. 5 is a more detailed circuit diagram of a CDS circuit according tosome example embodiments of inventive concepts;

FIG. 6 is a schematic cross-sectional view of a transistor according tosome example embodiments of inventive concepts;

FIG. 7 is a schematic cross-sectional view of a transistor according tosome example embodiments of inventive concepts;

FIG. 8 is a schematic cross-sectional view of a transistor according tosome example embodiments of inventive concepts;

FIG. 9 is a timing diagram illustrating an operation of a CDS circuitaccording to some example embodiments of inventive concepts;

FIG. 10 is a schematic block diagram of an image sensing systemincluding an image sensor according to some example embodiments ofinventive concepts; and

FIG. 11 is a block diagram of an electronic system including the imagesensor illustrated in FIG. 1 according to other example embodiments ofinventive concepts.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

FIG. 1 is a schematic block diagram of an image sensing system 1including an image sensor 100 according to an example embodiment ofinventive concepts. FIG. 2 is a detailed block diagram of the imagesensor 100 of FIG. 1.

Referring to FIGS. 1 and 2, the image sensing system 1 includes theimage sensor 100 and a digital signal processor 200.

The image sensing system 1 senses an image of an object 400 captured viaa lens 500 under control of the digital signal processor 200. Thedigital signal processor 200 may output the image sensed by and outputfrom the image sensor 100 to a display unit 300. Examples of the displayunit 300 include various devices capable of outputting an image. Forexample, the display unit 300 may be a computer or, a mobile phone, butis not limited thereto.

The digital signal processor 200 includes a camera control 210, an imagesignal processor 220, a personal computer (PC) interface (IF) 230 and amemory 240. The camera control 210 controls a control register block175. The camera control 210 may control the image sensor 100(particularly, the control register block 175) by using aninter-integrated circuit (I²C) but example embodiments of inventiveconcepts are not limited thereto.

Each of the camera control 210, the image signal processor 220 and thepersonal computer (PC) interface (IF) 230 may be implemented inhardware, a processor configured to execute software, firmware, or anycombination thereof, for example. When at least one of the cameracontrol 210, the image signal processor 220 and the personal computer(PC) interface (IF) 230 is hardware, such existing hardware may includeone or more Central Processing Units (CPUs), digital signal processors(DSPs), application-specific-integrated-circuits (ASICs), fieldprogrammable gate arrays (FPGAs) computers or the like configured asspecial purpose machines to perform the functions of the at least one ofthe camera control 210, the image signal processor 220 and the personalcomputer (PC) interface (IF) 230. CPUs, DSPs, ASICs and FPGAs maygenerally be referred to as processors and/or microprocessors.

In the event where at least one of the camera control 210, the imagesignal processor 220 and the personal computer (PC) interface (IF) 230is a processor executing software, the processor is configured as aspecial purpose machine to execute the software, stored in a storagemedium (e.g., memory 240), to perform the functions of the at least oneof camera control 210, the image signal processor 220 and the personalcomputer (PC) interface (IF) 230. In such an embodiment, the processormay include one or more Central Processing Units (CPUs), digital signalprocessors (DSPs), application-specific-integrated-circuits (ASICs),field programmable gate arrays (FPGAs) computers.

The image signal processor 220 receives image data which is an outputsignal of a buffer 190, processes/handles the image data so that it maybe visible to human eyes, and outputs the processed/handled image datato the display unit 300 via the PC IF 230.

Although FIG. 1 illustrates that the image signal processor 220 isincluded in the digital signal processor 200, the position of the imagesignal processor 220 may be in another location. For example, the imagesignal processor 220 may be included in the image sensor 100.

The image sensor 100 includes a pixel array 110, a row driver 120, ananalog-to-digital converter (hereinafter referred to as ‘ADC’) 130, aramp signal generator 155, a timing generator 165, a control registerblock 175, and the buffer 190.

The pixel array 110 may include a plurality of pixels (e.g., pixels 111)in the form of a matrix, in which each of the plurality of pixels isconnected to one of a plurality of rows and one of a plurality ofcolumns.

The pixels 111 may include a red filter which transmits light of a redwavelength band, a green filter which transmits light of a greenwavelength band, and a blue filter which transmits light of a bluewavelength band. In some example embodiments, the pixels 111 may includea cyan filter, a magenta filter, and a yellow filter.

Each of the pixels 111 includes a plurality of transistors and aphotosensitive device (e.g., a photodiode or a pinned photodiode). Eachof the pixels 111 senses light by using the photosensitive device, andconverts the light into an electrical signal to generate an imagesignal.

The timing generator 165 may control operations of the row driver 120,the ADC 130, and the ramp signal generator 155 by outputting a controlsignal to the row driver 120, the ADC 130, and the ramp signal generator155. The control register block 175 may control operations of the rampsignal generator 155, the timing generator 165, and the buffer 190 byoutputting a control signal to the ramp signal generator 155, the timinggenerator 165, and the buffer 190. In this case, the control registerblock 175 is operated under control of the camera control 210. Thecamera control 210 may be embodied by hardware or hardware executingsoftware.

The row driver 120 drives the pixel array 110 in units of the rows. Forexample, the row driver 120 may generate a row selection signal. Thatis, the row driver 120 may decode a row control signal (e.g., an addresssignal) generated by the timing generator 165, and select at least oneamong row lines of the pixel array 110 in response to the decoded rowcontrol signal. The pixel array 110 outputs, to the ADC 130, a resetsignal and an image signal from a row selected according to the rowselection signal provided from the row driver 120.

The ADC 130 includes a plurality of CDS circuits 140, a plurality ofcounters (e.g., counters 170), a plurality of memories (e.g., memories180), a column decoder 181, and a sense amplifier 183.

FIG. 3 is a more detailed block diagram of a CDS circuit 140 accordingto some example embodiments of inventive concepts. Referring to FIGS. 2and 3, the CDS circuit 140 may include a peripheral circuit 151 and acomparator 160.

The peripheral circuit 151 includes a pair of capacitors C1 and C2 and apair of switches SW1 and SW2.

The first capacitor C1 may be connected between a pixel signal inputnode IP and a first input node INN to compensate for an offset of thecomparator 160 and a variation in a pixel reset level. For example, thefirst capacitor C1 may block a direct-current (DC) of a received pixelsignal PIX and output a resultant signal to the comparator 160. The DCblocking may be understood as removing a DC component of a signal.

The first switch SW1 may be connected between the first input node INNand an output node OC to control an operation of the CDS circuit 140.

The second capacitor C2 may be connected between a ramp signal inputnode IR and a second input node INP to compensate for an offset of thecomparator 160 and a variation in a ramp level. For example, the secondcapacitor C2 may block DC of a received ramp signal RAMP and output aresultant signal to the comparator 160.

The second switch SW2 may be connected between the second input node INPand a comparison node R1 to control a CDS operation. The first switchSW1 or the second switch SW2 may be controlled by a switch signal SW.The switch signal SW may be generated by the timing generator 165.

As illustrated in FIG. 9, the pixel signal PIX may include a resetsignal RST or an image signal SIG.

The comparator 160 may compare an initial DC voltage of the pixel signalPIX output from the pixel 111 and an initial DC voltage of the rampsignal RAMP with each other, and output a comparison signal COMPcorresponding to a result of comparing the initial DC voltages.

The first switch SW1 may connect the first input node INN of thecomparator 160 to the output node OC and the second switch SW2 mayconnect the second input node INP of the comparator 160 to thecomparison node R1, in response to the switch signal SW activated in anauto-zero period.

For example, the output node OC may be a positive output terminal of thecomparator 160 and the comparison node R1 may be a negative outputterminal of the comparator 160.

When the first input node INN of the comparator 160 is connected to theoutput node OC, levels of the pixel signal PIX stored in the firstcapacitor C1 and the comparison signal COMP become the same and thusreset noise of the pixel signal PIX and an offset of the comparator 160may be removed.

FIG. 4 is a more detailed circuit diagram of a CDS circuit 140 accordingto some example embodiments of inventive concepts. The CDS circuit 140and a comparator 160 illustrated in FIG. 4 are respectively embodimentsof the CDS circuit 140 and the comparator 160 illustrated in FIG. 3.

Referring to FIGS. 3 and 4, the CDS circuit 140 may include thecomparator 160 and a peripheral circuit 151.

The comparator 160 may include a current source I and first to fourthtransistors N1, N2, P1, and P2.

According to some example embodiments, the comparator 160 may includeone or more operational transconductance amplifier (OTA) circuits. Asillustrated in FIG. 4, the OTA circuits refer to a circuit including aplurality of transistors, a current-mirror circuit, and the currentsource I (for example, the comparator 160 of FIG. 4).

FIG. 4 illustrates the comparator 160 including one OTA circuit and theCDS circuit 140 including the comparator 160, but example embodiments ofinventive concepts are not limited thereto.

According to some example embodiments, the comparator 160 may includetwo OTA circuits. In this case, the comparator 160 may include a firstOTA circuit and a second OTA circuit. The first OTA circuit may besubstantially the same as the comparator 160 of FIGS. 4 and 5 and thesecond OTA circuit may be embodied as an inverter to amplify an outputsignal of the first OTA circuit and output a comparison signal COMPwhich is an output signal of the comparator 160.

It will be hereinafter assumed that the comparator 160 includes one OTAcircuit in a description with reference to FIGS. 4 and 5 below. Thecomparator 160 may compare a pixel signal PIX and a ramp signal RAMPwith each other, and output the comparison signal COMP corresponding toa result of comparing the pixel signal PIX and the ramp signal RAMP witheach other.

The first transistor P1 and the second transistor P2 may be each a PMOStransistor. A third transistor N1 and a fourth transistor N2 may be eacha NMOS transistor.

For example, the first transistor P1 and the second transistor P2 mayform a current-mirror circuit.

The first transistor P1 may be connected between a voltage generator VDDand an output node OC, and a gate thereof may be connected to acomparison node R1.

The second transistor P2 may be connected between the voltage generatorVDD and the comparison node R1, and a gate thereof may be connected tothe comparison node R1.

The third transistor N1 may be connected between a common node CN andthe output node OC and a gate thereof may be connected to a first inputnode INN.

The fourth transistor N2 may be connected between the common node CN andthe comparison node R1 and a gate thereof may be connected to a secondinput node INP.

The current source I may be connected between an earth terminal and thecommon node CN.

Referring to FIGS. 3 and 4, a peripheral circuit 151 may include a firstswitch SW1, a second switch SW2, a first capacitor C1, and a secondcapacitor C2.

The first and second switches SW1 and SW2 may be embodied astransistors. For example, the first switch SW1 and the second switch SW2may be PMOS transistors.

As described above, the first capacitor C1 may be connected between apixel signal input node IP and the first input node INN. The firstswitch SW1 may be connected between the first input node INN and acomparison signal output node OC. The second capacitor C2 may beconnected between a ramp signal input node IR and the second input nodeINP. The second switch SW2 may be connected between the second inputnode INP and the comparison node R1.

The output node OC may output the comparison signal COMP.

Unlike that illustrated in FIG. 4, the current source I may be connectedbetween the voltage generator VDD and the first and second transistorsP1 and P2, but example embodiments of inventive concepts are not limitedthereto.

The first to fourth transistors P1, P2, N1, and N2 may have a differenttransistor structure from those of the first and second switches SW1 andSW2, as will be described in detail with reference to FIGS. 6 to 8below.

FIG. 5 is a more detailed circuit diagram of a CDS circuit according tosome example embodiments of inventive concepts.

The CDS circuit of FIG. 5 will be described focusing on the differencesfrom the CDS circuit 140 of FIG. 4 to avoid redundant description.

Referring to FIG. 5, a peripheral circuit 151 may include a first switchSW1, a second switch SW2, a first capacitor C1, and a second capacitorC2.

The first switch SW1 may include a plurality of first switch devicesSE11 to SE1 n. Here, n denotes an integer which is greater than or equalto ‘2’. The second switch SW2 may include a plurality of second switchdevices SE21 to SE2 n. Although FIG. 5 illustrates that the plurality offirst switch devices SE11 to SE1 n and the plurality of second switchdevices SE21 to SE2 n are the same in number, example embodiments ofinventive concepts are not limited thereto.

Each of the plurality of first switch devices SE11 to SE1 n may beembodied as a transistor. For example, each of the plurality of firstswitch devices SE11 to SE1 n may be a PMOS transistor.

Each of the plurality of second switch devices SE21 to SE2 n may beembodied as a transistor. For example, each of the plurality of secondswitch devices SE21 to SE2 n may be a PMOS transistor.

The first to fourth transistors P1, P2, N1, and N2 may have a differenttransistor structure from those of the plurality of first switch devicesSE11 to SE1 n and the plurality of second switch devices SE21 to SE2 n,as will be described in detail with reference to FIGS. 6 to 8 below.

Referring to FIG. 5, the first capacitor C1 may be influenced by onlyone switch device (e.g., the first switch device SE11) among theplurality of first switch devices SE11 to SE1 n.

Similarly, the second capacitor C2 may be influenced by only one switchdevice (e.g., the second switch device SE21) among the plurality ofsecond switch devices SE21 to SE2 n.

Thus, when the amount of interface-trapped charges is decreased throughthe first switch device SE11 and the second switch device SE21, thefirst and second capacitors C1 and C2 may be more rapidly stabilized.

FIG. 6 is a schematic cross-sectional view of a transistor according tosome example embodiments of inventive concepts. FIG. 7 is a schematiccross-sectional view of a transistor according to some exampleembodiments of inventive concepts. FIG. 8 is a schematic cross-sectionalview of a transistor according to some example embodiments of inventiveconcepts.

In FIGS. 6 to 8, only main parts of the transistors are illustrated tobriefly describe inventive concepts. Thus, some elements of thetransistors may be omitted here.

The transistor illustrated in FIG. 6 may be substantially the same asthe first to fourth transistors P1, P2, N1, and N2 of FIG. 4 or 5.

The transistor illustrated in FIG. 7 may be substantially the same asthe first and second switches SW1 and SW2 of FIG. 4.

The transistor illustrated in FIG. 8 may be substantially the same asthe first and second switch devices SE1 and SE2 of FIG. 5 or the firstand second switches SW1 and SW2 of FIG. 4.

For convenience of explanation, the transistor of FIG. 6 is described asa PMOS transistor, but example embodiments of inventive concepts are notlimited thereto. For example, the structure of FIG. 6 may be an NMOStransistor.

Referring to FIGS. 6 to 8, the transistor may include a gate 10, a gateinsulating layer 20, a silicon-germanium (hereinafter referred to as‘cSiGe’) layer 30, a source 40, a drain 50, and a substrate 80.

As generally known, when a gate voltage is applied to the gate 10, achannel may be formed between the source 40 and the drain 50, via whichelectric charges move. In this case, the length L of the channel isillustrated as the length between the source 40 and the drain 50 butexample embodiments of inventive concepts are not limited thereto.

As the length L of the channel increases, interface charge trapping mayoccur more frequently.

The transistor may further include a low-doped region (not shown) and ahalo region (not shown) between the source 40 and the drain 50.

The halo region may be a region formed to prevent a threshold voltagefrom decreasing when the length L of the channel decreases.

The cSiGe layer 30 may be formed above the substrate 80 of thetransistor.

The cSiGe layer 30 may be a layer formed according to an epitaxialgrowth process to improve the performance of the PMOS transistor.

The gate insulating layer 20 may be formed on the cSiGe layer 30.

The gate 10 may be formed on the gate insulating layer 20.

A gate protective film (not shown) may be formed on sidewalls of thegate 10 to protect the gate 10.

When the transistor is a PMOS transistor, ions may be implanted into anN well in the substrate 80.

P+ ion implanted regions, i.e., the source 40 and the drain 50, may beformed at opposite sides of the gate 10 by implanting P+ ions thereinto.

The cSiGe layer 30 may decrease the threshold voltage but increasesinterface roughness with respect to other layers (e.g., the substrate 80and the gate insulating layer 20). An increase in the interfaceroughness may result in a decrease in the reliability and performance ofthe transistor. For example, the cSiGe layer 30 may cause interfacecharge trapping to occur in the transistor.

Thus, in order to decrease interface charge trapping, a transistor inwhich the length L of the channel is decreased or from which the cSiGelayer 30 is removed may be used as a first switch or a second switch.

FIG. 7 is a schematic cross-sectional view of a transistor in which thecSiGe layer 30 is omitted on the basis of the above principle, accordingto other example embodiments of inventive concepts. Thus, the transistorof FIG. 7 may be substantially the same as the first switch SW1 or thesecond switch SW2 illustrated in FIG. 4.

FIG. 8 is a cross-sectional view of a transistor in which the length L′of a channel is shorter than the length L of the channel illustrated inFIG. 6, based on the above principle. That is, referring to FIGS. 6 and8, the length L′ of the channel of the transistor illustrated in FIG. 8may be shorter than the length L of the channel of the transistor ofFIG. 6.

The transistor of FIG. 8 is illustrated as including a cSiGe layer 30but example embodiments of inventive concepts are not limited thereto.

The transistor of FIG. 8 may be substantially the same as the first andsecond switches SW1 and SW2 of FIG. 4. Alternatively, the transistor ofFIG. 8 may be substantially the same as the first and second switchdevices SE1 and SE2 of FIG. 5.

FIG. 9 is a timing diagram illustrating an operation of a CDS circuitaccording to some example embodiments of inventive concepts.

It is assumed that a range of a pixel signal PIX and a range of a rampsignal RAMP are within an input range of the comparator 160.

Referring to FIG. 9, a switch signal SW may change from a low level to ahigh level, i.e., this signal may change to be ‘on’, at a point of timeT0.

The ramp signal RAMP may have a starting voltage level SL. A resetsignal RST included in the pixel signal PIX may have a reset level RL.

Referring to FIGS. 1 to 4, in a period T0 to T1, since the switch signalSW is ‘on’, the first input node INN and the output node OC may beconnected to each other and the second input node INP and the comparisonnode R1 may be connected to each other.

The timing controller 165 may control the switch signal SW.

At a point of time T1, the switch signal SW may change from the highlevel to the low level, i.e., this signal may change to be ‘off’.

A period T1 to T2 is referred to as a settling time ST. The settlingtime ST may be understood as a time to stabilize the CDS circuit 140.

In a period T0 to T2, a comparison signal COMP may be a signalcorresponding to the difference between the ramp signal RAMP having thestarting voltage level SL and the pixel signal PIX having the resetlevel RL (e.g., the reset signal RST). For example, the starting voltagelevel SL and the reset level RL may be substantially the same.

The settling time ST may be understood as a time for the ramp signalRAMP to change to an offset level OL after a switch is ‘off’ accordingto the switch signal SW as illustrated in FIG. 9. However, exampleembodiments of inventive concepts are not limited thereto.

The settling time ST may increase due to either a potential barrier ofan energy band caused by implantation of a halo-region dopant materialor an interface trap.

According to an example embodiment of inventive concepts, the settlingtime ST may be prevented from increasing in units of several μs due tothe interface trap.

When the settling time ST decreases, a time to perform a whole ADCoperation decreases to enable an HFR operation of the image sensingsystem 1.

At a point of time T2, the ramp signal RAMP increases by a certainoffset and thus changes from the starting voltage level SL to the offsetlevel OL. The offset level OL may be higher than the reset level RL.

In a period T2 to T3, the comparison signal COMP may correspond to thedifference between the ramp signal RAMP having the offset level OL andthe pixel signal PIX having the reset level RL, e.g., the reset signalRST.

At a point of time T3, a ramp enable signal may be supplied to the rampsignal generator 155 to activate the ramp signal RAMP.

In this case, the activation of the ramp signal RAMP means that the rampsignal RAMP decreases by a certain degree, starting from the offsetlevel OL.

In a period T3 to T4, the comparison signal COMP may correspond to thedifference between the ramp signal RAMP decreasing by the certain degreeand the pixel signal PIX having the reset level RL, e.g., the resetsignal RST.

In a period T4 to T5, the pixel signal PIX may change from the resetlevel RL to a signal level GL. In a period T4 to T5, the comparisonsignal COMP may correspond to the difference between the ramp signalRAMP decreasing by the certain degree and the pixel signal PIX havingthe signal level GL, e.g., the image signal SIG.

At a point of time T5, the ramp signal RAMP may change to the offsetlevel OL.

In a period T5 to T6, the comparison signal COMP may correspond to thedifference between the ramp signal RAMP having the offset level OL andthe pixel signal PIX having the signal level GL, e.g., the image signalSIG.

At a point of time T6, the ramp enable signal may be supplied to theramp signal generator 155 to activate the ramp signal RAMP.

In a period T6 to T7, the comparison signal COMP may correspond to thedifference between the ramp signal RAMP decreasing by the certain degreeand the pixel signal PIX having the signal level GL, e.g., the imagesignal SIG.

FIG. 10 is a schematic block diagram of an image sensing systemincluding an image sensor according to some example embodiments ofinventive concepts. Referring FIG. 10, the image sensing system 1000 maybe implemented by a data processing apparatus, such as a mobile phone, apersonal digital assistant (PDA), a portable media player (PMP), or asmart phone that can use or support the Mobile Industry ProcessorInterface (MIPI). The image sensing system 1000 includes an applicationprocessor 1010, an image sensor 1040, and a display 1050.

A camera serial interface (CSI) host 1012 included in the applicationprocessor 1010 performs serial communication with a CSI device 1041included in the image sensor 1040 through CSI. For example, an opticalde-serializer (DES) may be implemented in the CSI host 1012, and anoptical serializer (SER) may be implemented in the CSI device 1041. Theimage sensor 1040 corresponds to the image sensor 100 described in FIG.1 through FIG. 9.

A display serial interface (DSI) host 1011 included in the applicationprocessor 1010 performs serial communication with a DSI device 1051included in the display 1050 through DSI. For example, an opticalserializer may be implemented in the DSI host 1011, and an opticalde-serializer may be implemented in the DSI device 1051.

The image sensing system 1000 may also include a radio frequency (RF)chip 1060 which communicates with the application processor 1010. Aphysical layer (PHY) 1013 of the image sensing system 1000 and a PHY ofthe RF chip 1060 communicate data with each other according to a MINDigRF standard. The image sensing system 1000 may further include atleast one element among a GPS 1020, a storage device 1070, a microphone1080, a dynamic random access memory (DRAM) 1085 and a speaker 1290. Theimage sensing system 1000 may communicate using Wimax (WorldInteroperability for Microwave Access) 1030, WLAN (Wireless local areanetwork) 1033 or UWB (Ultra Wideband) 1036, etc.

FIG. 11 is a block diagram of an electronic system 1100 including theimage sensor 100 illustrated in FIG. 1 according to other exampleembodiments of inventive concepts. Referring to FIGS. 1 and 11, theelectronic system 1100 may include the image sensor 100, a processor1110, a memory 1120, a display unit 1130, and an I/F 1140.

The processor 1110 may control the operation of the image sensor 100.The processor 1110 may generate a two or three dimensional image basedon depth information and color information (e.g., at least one among redinformation, green information, blue information magenta information,cyan information, and yellow information) from the image sensor 100.

The memory 1120 may store a program for controlling the operation of theimage sensor 100 through a bus 1150 according to the control of theprocessor 1110 and may also store the image. The processor 1110 mayaccess the memory 1120 and execute the program. The memory 1120 may beformed as a non-volatile memory.

The image sensor 100 may generate two or three dimensional imageinformation based on a digital pixel signal (e.g., color information ordepth information) under the control of the processor 1110.

The display unit 1130 may receive the image from the processor 1110 orthe memory 1120 and display the image on a display (e.g., a liquidcrystal display (LCD) or an active-matrix organic light emitting diode(AMOLED) display). The I/F 1140 may be formed for the input and outputof the two or three dimensional image. The I/F 1140 may be implementedas a wireless I/F.

Inventive concepts can also be embodied as computer-readable codes on acomputer-readable medium. The computer-readable recording medium is anydata storage device that can store data as a program which can bethereafter read by a computer system. Examples of the computer-readablerecording medium include read-only memory (ROM), random-access memory(RAM), CD-ROMs, magnetic tapes, floppy disks, and optical data storagedevices.

The computer-readable recording medium can also be distributed overnetwork coupled computer systems so that the computer-readable code isstored and executed in a distributed fashion. Also, functional programs,codes, and code segments to accomplish inventive concepts can be easilyconstrued by programmers.

According to various example embodiments of inventive concepts, a CDScircuit employs a transistor having no cSiGe layer as an auto-zeroswitch, unlike transistors configured as comparators. Accordingly,interface roughness may decrease, thereby decreasing charge trapping.

Furthermore, in an image sensor including a CDS circuit according to anexample embodiment of inventive concepts, charge trapping decreases,thereby decreasing a pixel settling time.

In an image sensing system including an image sensor according to anexample embodiment of inventive concepts, a HFR may be realized todecrease column fixed pattern noise (CFPN).

While inventive concepts have been particularly shown and described withreference to the example embodiments illustrated in the drawings, theseembodiments are merely examples. It would be obvious to those ofordinary skill in the art that these embodiments are to cover allmodifications, equivalents, and alternatives falling within the scope ofthe inventive concept. Accordingly, the technical scope of inventiveconcepts should be defined based on the technical idea of the appendedclaims.

What is claimed is:
 1. A correlated double sampling (CDS) circuitcomprising: a comparator configured to compare a pixel signal and a rampsignal with each other and output a comparison signal corresponding to aresult of comparing the pixel signal and the ramp signal with eachother; and a first switch and a second switch, each of the first switchand the second switch configured to remove an offset component of thecomparator, wherein the first switch and the second switch areconfigured to decrease a settling time between when switching operationsof the first switch and the second switch are performed and whenanalog-digital conversion (ADC) is performed.
 2. The CDS circuit ofclaim 1, wherein the first switch is configured to connect a first inputnode and an output node to each other, the second switch is configuredto connect a second input node and a comparison node to each other, thecomparator is configured to receive the pixel signal via the first inputnode, and the comparator is configured to receive the ramp signal viathe second input node.
 3. The CDS circuit of claim 2, wherein thecomparator comprises: a first transistor operably connected between avoltage generator and the output node and including a gate connected tothe comparison node; a second transistor operably connected between thevoltage generator and the comparison node and including a gate connectedto the comparison node; a third transistor operably connected between acommon node and the output node and including a gate connected to thefirst input node; and a fourth transistor operably connected between thecommon node and the comparison node and including a gate connected tothe second input node, wherein to the gate of the third transistor isconfigured to receive the pixel signal, and the gate of the fourthtransistor is configured to receive the ramp signal.
 4. The CDS circuitof claim 3, wherein each of the first to fourth transistors and thefirst and second switches is a transistor comprising: a substrate of thetransistor; a gate insulating layer formed on the substrate; a gateformed on the gate insulating layer; and a source and a drain formed atopposite sides of the gate by implanting ions, wherein each of the firstto fourth transistors further comprises a silicon-germanium (cSiGe)layer formed between the substrate and the gate insulating layer.
 5. TheCDS circuit of claim 1, wherein the first switch and the second switchare transistors to which a silicon-germanium (cSiGe) process is notapplied.
 6. The CDS circuit of claim 3, wherein the first switchcomprises a plurality of first switch devices operably connected inseries, and the second switch comprises a plurality of second switchdevices operably connected in series.
 7. The CDS circuit of claim 6,wherein each of the first to fourth transistors, the plurality of firstswitch devices, and the plurality of second switch devices is atransistor comprising: a substrate of the transistor; a gate insulatinglayer formed on the substrate; a gate formed on the gate insulatinglayer; and a source and a drain formed at opposite sides of the gate byimplanting ions, wherein each of the first to fourth transistors furthercomprises a silicon-germanium (cSiGe) layer formed between the gateinsulating layer and the substrate.
 8. The CDS circuit of claim 3,wherein each of the first to fourth transistors and the first and secondswitches is a transistor comprising: a substrate of the transistor; agate insulating layer formed on the substrate; a gate formed on the gateinsulating layer; and a source and a drain formed at opposite sides ofthe gate by implanting ions, wherein the first switch and the secondswitch are transistors having a length of a channel which is shorterthan lengths of channels of the first to fourth transistors, wherein thelength of the channel is equal to a horizontal distance between thesource and the drain.
 9. The CDS circuit of claim 1, further comprising:a first capacitor operably connected to a pixel node and a first inputnode; and a second capacitor operably connected to a ramp node and asecond input node, wherein the first capacitor and the second capacitorare configured to block a direct-current of the pixel signal and theramp signal.
 10. The CDS circuit of claim 3, wherein the first switch,the second switch, the third transistor, and the fourth transistor areP-MOS transistors, and the first transistor and the fourth transistorare N-MOS transistors.
 11. An image sensor including a pixel arrayhaving a plurality of pixels for outputting a pixel signal, a rampsignal generator for outputting a ramp signal, and a correlated doublesampling (CDS) circuit, wherein the CDS circuit comprises: a firstcapacitor configured to receive a pixel signal; a second capacitorconfigured to receive a ramp signal; a comparator configured to comparethe pixel signal and the ramp signal with each other and output acomparison signal corresponding to a result of comparing the pixelsignal and the ramp signal with each other; and a first switch and asecond switch, each of the first switch and the second switch configuredto remove an offset component of the comparator, wherein the firstswitch and the second switch are configured to decrease a settling timebetween when switching operations of the first switch and the secondswitch are performed and when analog-digital conversion (ADC) isperformed, and the first switch and the second switch are transistors towhich a silicon-germanium (cSiGe) process is not applied.
 12. The imagesensor of claim 11, wherein the first switch is configured to connect afirst input node and an output node to each other, the second switch isconfigured to connect a second input node and a comparison node to eachother, the comparator is configured to receive the pixel signal via thefirst capacitor, the first capacitor operably connected to the firstinput node, and the comparator is configured to receive the ramp signalvia the second capacitor, the second capacitor operably connected to thesecond input node.
 13. The image sensor of claim 12, wherein thecomparator comprises: a first transistor operably connected between avoltage generator and the output node and including a gate connected tothe comparison node; a second transistor operably connected between thevoltage generator and the comparison node and including a gate connectedto the comparison node; a third transistor operably connected between acommon node and the output node and including a gate connected to thefirst input node; and a fourth transistor operably connected between thecommon node and the comparison node and including a gate connected tothe second input node.
 14. The image sensor of claim 13, wherein each ofthe first to fourth transistors and the first and second switches is atransistor comprising: a substrate of the transistor; a gate insulatinglayer formed on the substrate; a gate formed on the gate insulatinglayer; and a source and a drain formed at opposite sides of the gate byimplanting ions, and each of the first to fourth transistors furthercomprises a silicon-germanium (cSiGe) layer formed between the substrateand the gate insulating layer.
 15. The image sensor of claim 11, whereinthe first capacitor and second capacitor are configured to blockdirect-current of the pixel signal and the ramp signal.